1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus such as a copier, a facsimile, and a printer, and more particularly, to an image forming apparatus including a corona charger.
2. Discussion of the Background
A corotron charger is known as an example of a corona charger. A corotron charger is formed of a metal case made of, for example, aluminum, having a cylindrical shape or a square-cylindrical shape with an open portion.
In substantially a center of the metal case, tungsten wires having a diameter of approximately 50 μm are suspended as a corona electrode.
Direct-current (DC) voltage, alternating current (AC) voltage, or AC voltage superimposed on DC voltage is applied to the corona electrode. DC voltage or ground voltage (0V) is applied to the metal case. When such voltages are applied to the corona electrode and the metal case, a so-called corona discharge occurs.
A device to be charged is disposed facing the opening of the metal case. Ions generated during corona discharge are supplied to the device so that the surface thereof is charged. An advantage of the corotron charger is that its structure can be simple and economical.
However, in order to satisfy growing demand for an electrophotographic device capable of outputting high-quality images, when the corotron charger is used as a charger for charging a photoreceptor serving as an image carrier which carries electrophotography, charge uniformity tends to be difficult to achieve.
For this reason, the corotron charger is less likely to be used as a main charger of the photoreceptor. Instead, the corotron charger is used for purposes such as discharge and adjustment of toner charging polarity.
A scorotron charger including a grid electrode is known as a corona charger having good charge uniformity compared to the corotron charger. The grid electrode is provided between the corona electrode and the photoreceptor as a device to be charged and is supplied with DC voltage or a grid voltage.
The scorotron charger is constructed in such a way that a part of corona discharge flowing from the corona electrode to the photoreceptor is supplied to the grid electrode to which the grid voltage is applied, so that the flow of the corona discharge flowing from the corona electrode to the photoreceptor is controlled. Accordingly, the photoreceptor is charged to a substantially similar if not identical potential as the grid voltage.
Therefore, this structure is advantageous in that it is easy to control the charge potential of the photoreceptor, and to achieve charge uniformity.
In order to maintain the grid electrode at a certain potential, a method in which a constant voltage passive component serving as a self-bias voltage applicator is connected to the grid electrode is known.
Compared to the corotron charger, this type of scorotron charger is advantageous in that the charge potential of the photoreceptor is easy to control.
However, in a case of a direct-current (DC) type scorotron charger, in which DC voltage is applied to the corona electrode, a potential fluctuation of approximately 10 to 50 V is likely to occur due to variations in atmospheric temperature and humidity.
Consequently, charge uniformity is not sufficient, and thus enhancement of charge uniformity is further desired.
In addition, in the scorotron charger, there is corona discharge which flows to the grid electrode rather than to the photoreceptor. Thus, compared to the corotron charger, the scorotron charger is inefficient.
Furthermore, when using the direct-current (DC) type scorotron charger for an extended period of time, the grid electrode is exposed to the corona discharge for an extended period of time. Consequently, the surface of the grid electrode is degraded, and/or a foreign matter adheres to the surface of the grid electrode.
The deterioration of the grid electrode surface causes the voltage to decrease so that the effective voltage of the grid electrode changes, resulting in fluctuation of the charge potential.
Consequently, uniformity of the charge potential of the photoreceptor is not sufficient over time.
In light of the above, use of an alternating-current (AC) type scorotron charger has been considered.
In the AC-type scorotron charger, the grid electrode is provided between the corona electrode and the photoreceptor as a device to be charged and is supplied with DC voltage or a grid voltage. The voltage, including the AC component with AC voltage superimposed on DC voltage, is applied to the corona electrode.
In the AC-type scorotron charger, the above-described drawbacks associated with the DC-type scorotron charger are remedied. Accordingly, a charger with good charge uniformity is achieved.
However, the AC-type scorotron charger has a drawback, in that high-speed charging ability is degraded because positive and negative corona discharges alternatively occur.
High-speed charging ability herein refers to a maximum moving speed of the photoreceptor which the charger can accommodate to charge to a desired charge potential when the traveling speed of the photoreceptor increases. High-speed charging ability may refer to a minimum width of an opening of the charger at which the charger can charge to a desired charge potential.
In order to remedy deterioration of high-speed charging ability in the AC-type scorotron charger, the voltage applied to the charger may be increased, or a width of an opening in the surface moving direction of the photoreceptor may be increased.
However, an increase in the applied voltage may cause the size and the cost of a power source to increase. Consequently, the size and the cost of an entire apparatus may increase accordingly.
Furthermore, when the width of the opening of the charger increases, a ratio of the charger to a circumference of the photoreceptor increases, increasing the size of the apparatus and limiting flexibility of parts allocation and location of devices near the photoreceptor.
In order to rectify the above-described problems, JP-H05-2988-A, for example, proposes a DC/AC double charger consisting of an AC-type scorotron charger connected to a downstream portion of a DC-type corotron or scorotron charger.
According to such a related art charger, the DC-type corotron or scorotron charger charges the photoreceptor to a certain level, and the AC-type scorotron charger evenly charges the photoreceptor. By taking advantage of characteristics of both the AC-type and DC-type chargers, a corona charger which satisfies both high-speed charging ability and the charge uniformity is attained.
However, the DC/AC double charger has a drawback, insofar as the width of opening of the charger further increases. In general, in order to stably generate the corona discharge, it is necessary to dispose the case and the grid electrode 5 to 10 mm away from the discharge electrode in the corona discharge charger.
Consequently, one side of the case needs to have a length of 10 to 20 mm. According to JP-H05-2988-A, two such charges having a similar if not identical size are needed, thereby doubling the space occupied by the two chargers near the photoreceptor.
As a result, such problems may arise as the size of the apparatus increases, and flexibility of parts allocation and the location of devices near the photoreceptor is significantly reduced. Furthermore, when there are more types of applied voltage, the size and the cost of the power source may increase, thereby causing the size and the cost of the entire apparatus to increase as well.
In order to facilitate an understanding of the background art, a description will now be given of an image forming apparatus such as a copier and a printer for a color image using electrophotographic processing that employs the above-described corona charger as a device for charging the photoreceptor.
Various types of image forming apparatuses forming a color image by overlapping a plurality of toner images on a photoreceptor are disclosed, for example, in JP-S63-172286-A, and JP-3646278-B and JP-3385008-B. The process employed by these image forming apparatuses is a so-called photoreceptor color overlapping method.
One example of such an image forming apparatus is equipped with a charger, an exposure unit, a plurality of developing units, a transfer unit, and a cleaning unit disposed around a photoreceptor. In the image forming apparatus, charging, exposure, and development are repeatedly performed for each different color so that a plurality of toner images are overlaid on one another on the same area of the photoreceptor.
Subsequently, the toner images are transferred onto a transfer sheet all at once and are fixed to produce a color image.
The image forming apparatus using the photoreceptor color overlapping method includes a single photoreceptor, and does not use an intermediate transfer member. Thus, the image forming apparatus of this type contributes to space saving and resource reduction.
In the image forming apparatus disclosed in JP-3646278-B, a charger, an exposure portion, and a developing unit are provided for each color around the photoreceptor.
Without rotating the photoreceptor for a number of times, a plurality of colors is overlaid on the photoreceptor. Therefore, the image forming apparatus of this type is advantageous in that high-speed image formation is made possible without reducing image forming speed.
However, in the image forming apparatus disclosed in JP-3646278-B, since the charger, the exposure portion, and the developing unit are provided for each color around the photoreceptor, there is not much room around the photoreceptor. Thus, reduction of the size of the charger is an important issue in order to reduce the space occupied by the charger.
In the image forming apparatus of the photoreceptor color overlapping method, a non-contact type developing unit for a non-contact developing method is used.
In such a non-contact type developing unit for the non-contact developing method, a developer on a toner carrier and a photoreceptor are disposed facing each other in a non-contacting manner so that the toner image formed on the photoreceptor is not distorted during the subsequent developing process, in which a toner image to be overlapped later is developed.
Even if the non-contact developing unit is used, however, the amount of toner to be overlaid on the photoreceptor is reduced due to the toner image already being formed on the photoreceptor, leading to a reduction in the amount of toner to be overlapped.
One cause of this problem may be that the size of the electrical charge in the toner layer formed on the photoreceptor causes the potential after exposure not to decrease as much as the non-toner adhesion area. Consequently, the potential after exposure rises.
In order to solve this problem, JP-2782872-B proposes using a DC/AC-double charger consisting of an AC-type scorotron charger connected to a downstream portion of a DC-type scorotron charger during a process in which the photoreceptor is charged to a desired potential after the toner image of the first color is formed thereon.
In the image forming apparatus, the DC-type scorotron charger of the DC/AC-double charger temporarily charges the photoreceptor to a potential higher than the given potential, and the AC component of the downstream portion of the AC-type scorotron charger reduces the potential to the desired potential. Accordingly, the potential of the toner layer is reduced.
When such a method is used, it is possible to reduce the amount of the electrical charge of the toner layer, and to reduce the reduction in the amount of toner adhered to the toner images overlaid on one anther.
As described above, the AC-type scorotron charger with a large width of the opening and the DC/AC-double charger are effective as chargers that facilitate both high-speed charging ability and charge uniformity. However, the space which the charger occupies is most likely large.
When such a charger is utilized in the image forming apparatus using the photoreceptor color overlapping method as described above, there is less space around the photoreceptor. Therefore, an increase in the size of the charger is a problem.